Acdisplay was based on motorola's active display before ambient display became a thing with lollipop's release. The app still plays nicely on an amoled screen with no dynamic background enabled in app and behaves pretty close to what ambient display does. That being said, not sure why you guys are so obsessed with this feature.
I quite liked it on the oneplus one so I miss using it moving into the op2. Just personal preference.